1. Field of the Invention
The present invention relates to a gas flow rate measuring device, in other words a gas flow meter.
2. Conventional Art
In order to control fuel supply to an internal combustion engine, it is necessary to grasp intake gas flow rate into the internal combustion engine, therefore, a gas flow rate measuring device which measures air flow rate in an intake air passage is disposed therein.
This gas flow rate measuring device is constituted by a bridge circuit including a heating resistor (also called as a hot wire), a gas temperature measuring resistor (also called as a cold wire) and two resistors having respective constant resistances and current supply to the bridge circuit is controlled to maintain the resistance balance of the bridge circuit so as to keep always the temperature of the heat resistor constant. With this structure, in response to an increase of the intake air flow rate in the air intake passage heat radiation amount from the heating resistor increases, thus the amount of current to be supplied for keeping the temperature of the heating resistor increases, thereby, the air flow rate can be measured based on a voltage appearing across the constant resistance resistor connected in series with the heating resistor. Then, if the voltage is processed by an adjustment processing circuit having a predetermined characteristic which shows a required air flow rate versus output signal characteristic, a flow rate signal having a predetermined relationship with the air flow rate can be outputted from the adjustment processing circuit.
Presently, analog signal processing circuits are primarily used for the adjustment processing circuit, however, if a digital signal processing circuit is used therefor, a highly accurate adjustment can be realized. Japanese patent No. 3073089 (which corresponds to JP-A-5-223611 (1993)) discloses an adjustment processing circuit in which the adjustment is performed by a digital signal processing circuit. More specifically, after converting a flow rate signal in a form of analog signal into a signal in a form of digital signal by an analog-digital conversion circuit, the adjustment processing circuit performs adjustment of zero point and span range through computing processing by a digital processing circuit, then the signal in a form of digital signal is converted into a flow rate signal in a form of analog signal by a digital-analog conversion circuit, thereafter, the flow rate signal in a form of analog signal corresponding to the desired gas flow rate is outputted. An adjustment coefficient necessary for executing the adjustment processing in the digital processing circuit is preserved in a memory device such as PROM. Further, with a digital processing circuit, since non-linear processing can be easily performed, when performing adjustment for an output, the adjustment of zero point and span range as well as non-linear adjustment can be easily realized. Through such non-linear adjustment the adjustment accuracy can be kept below xc2x12%.
Further, JP-A-11-118552 (1999) discloses an adjustment processing circuit in which, in order to perform adjustment in a form of digital signal an over sampling type analog-digital conversion circuit including a sigma delta modulator is used for an analog-digital conversion circuit, thereby, reduction in circuit scale thereof can be realized.
Still further, JP-A-2000-338193 discloses a digital processing circuit in which in order to perform adjustment in a form of digital signal an adjustment coefficient necessary for performing adjustment processing is written in a memory element such as PROM via a terminal of a digital input and output circuit for performing communication with outside of a sensor, and which proposes to use a 3rd degree polynomial for the adjustment processing.
Now, it is desirable that an error in output characteristic of a gas flow rate measurement device is small even when temperature varies, namely, temperature dependent error thereof is small. The temperature dependent error primarily includes two types of error, one is an intake air temperature dependent error which is caused when gas temperature varies under a constant circuit board temperature and the other is a circuit board temperature dependent error (also called as a module temperature dependent error) which is caused when the circuit board temperature varies under a constant gas temperature.
With regard to the intake air temperature dependent error, an output voltage V1 of a gas flow rate detection circuit is expressed by the following King""s equation defining a predetermined curve;
V1=(A+Bxc2x7Q1/2)1/2xe2x80x83xe2x80x83(1) 
wherein Q is a flow rate and A and B are constants and are functions of temperature, because these are ones containing thermal conductivity and kinetic viscosity of air, and inherently contain an intake air temperature dependent error.
For example, JP-A-8-278178 (1996) discloses a gas flow rate measurement device for decreasing the above intake air temperature dependent error in which a circuit is constituted so that the dependency of measurement error by temperature variation with respect to air flow rate is corrected by designing a temperature characteristic of a reference voltage generating circuit in the gas flow rate measuring device to have flow rate dependency which cancels out the measurement error.
Further, in the above referred to Japanese Patent No. 3073089, a flow rate signal in a form of analog signal outputted from the gas flow rate detection circuit is converted into a digital signal and intake air temperature is measured, then the digital flow rate signal is corrected from the measured intake air temperature by making use of a lookup table, thereby, the intake air temperature dependent error is reduced.
On the other hand, several adjusting methods of the board temperature characteristic have been proposed.
For example, JP-A-60-100218 (1985) proposes to constitute a reference voltage circuit for a zero span adjusting circuit constituted by a zener diode, a diode, an operational amplifier and a resistor and in which a temperature characteristic of the reference voltage is determined by adjusting in advance the value of current flowing through the zener diode by means of trimming the resistor, thereby, through properly setting the temperature characteristic of the reference voltage circuit, the board temperature dependent error is reduced substantially zero.
Further, JP-A-5-157602 (1993) discloses a use of a band gap voltage source circuit for the reference voltage circuit to thereby further decrease the board temperature dependent error.
Still further, JP-A-6-207842 (1994) proposes to operate a gas flow rate measuring device under a most preferable board temperature characteristic by setting the in-chips temperature at substantially constant to thereby reduce the error.
However, with the adjustment method in a form of digital signal which permits a highly accurate adjustment, since the flow rate signal is either digital-converted or analog-converted by making use of a reference voltage source as a reference voltage in both analog-digital conversion circuit and digital-analog conversion circuit, when the board temperature varies because of the temperature dependency of the reference voltage source, no negligible conversion error is caused during the analog-digital conversion and the digital-analog conversion.
Further, with the measure disclosed in JP-A-6-207842 (1994), it is difficult to keep the chip temperature accurately at a constant value, in addition since it takes time to raise the chip temperature such as immediately after making the power source, the board temperature dependent error during such interval can not be reduced.
An object of the present invention is to provide a gas flow rate measuring device which can reduce a temperature dependent error through adjustment in a form of digital signal.
Another object of the present invention is to provide a small sized gas flow rate measuring device which can reduce a temperature dependent error through adjustment in a form of digital signal.
Still another object of the present invention is to provide a small sized and easy manufacturable gas flow rate measuring device which can reduce a temperature dependent error through adjustment in a form of digital signal.
A gas flow rate measuring device according to the present invention which comprises a gas flow rate detection circuit which detects such as current flowing through a resistor disposed in a gas flow passage and voltage induced across the resistor dependent on the current flowing therethrough and outputs a flow rate signal in a form of analog signal in response to the gas flow rate flowing through the gas flow passage; a flow rate signal analog-digital conversion circuit which converts the flow rate signal in a form of analog signal outputted from the gas flow rate detection circuit into an electric signal in a form of digital signal; and an adjustment processing circuit which adjusts the flow rate signal in a form of digital signal so as to assume a desired output characteristic, is characterized in that the flow rate signal analog-digital conversion circuit and the adjustment processing circuit are integrated on a common semiconductor chip as well as a chip temperature sensor and a chip temperature signal analog-digital conversion circuit are also integrated on the common semiconductor chip, a chip temperature signal in a form of analog signal outputted from the chip temperature sensor is converted into a chip temperature signal in a form of digital signal by the chip temperature signal analog-digital conversion circuit and the converted digital chip temperature signal is inputted into the adjustment processing circuit so as to perform correction for reducing a temperature dependent error in a series of signal processing circuits.